Dipeptidyl peptidase IV (DPP-IV) is a multifunctional transmembrane glyco-protein that contains N-terminal dipeptidase activity. It is present on most mammalian cells, in a variety of tissues such as liver, kidney, small intestine, salivary gland, blood cells and plasma. Little is however known about the physiological role of DPP-IV.
DPP-IV has been implicated in cellular processes involving immune, inflammatory and endocrine functions. In vitro, DPP-IV has been shown to cleave many hormones and chemokines, such as e.g. glucagon-like peptide 1 (GLP-1).
GLP-1 is an incretin hormone that is released postprandially. GLP-1 has multifaceted actions, including glucose-induced stimulation of insulin biosynthesis and secretion, inhibition of glucagon secretion, regulation of gene expression, trophic effects on β cells, inhibition of food intake, and slowing of gastric emptying. These effects contribute to the normalisation of elevated blood glucose, as well as to the control of satiety and body weight. GLP-1 has been shown to reduce postprandial and fasting glycemia in subjects with type 2 diabetes mellitus and may therefore be a potentially useful new therapeutic agent in the treatment of type 2 diabetes mellitus. Moreover, GLP-1 could be used to increase satiety and also to prevent and treat obesity. See e.g. Conarello, S. L. et al. 2003, Proc. Nat. Acad. Sci. USA, vol. 100:6825-6830; Deacon, C. F. et al. 1998, Diabetes, vol. 47:764-769; Ahrén, B. et al. 2002, Diabetes Care, vol. 25:869-875; Näslund, E. et al. 1998, Am. J. Clin. Nutr., vol. 68:525-530; Meneilly, G. S. et al. 2003, Diabetes Care, vol. 26:2835-2841.
However, GLP-1 is rapidly degraded in plasma and therefore has a very short half-life of about 1-2 min. The enzyme mainly responsible for degradation of GLP-1 is DPP-IV. Inhibition of DPP-IV might therefore result in prolongation of the circulating half-life of GLP-1, such that GLP-1 levels increase as to be able to act as a therapeutic agent.
DPP-IV has also been shown to be involved in T cell activation and growth. In the immune system, DPP-IV is expressed primarily on the surface of T cells. It has been shown that the expression of DPP-IV is rapidly increasing upon mitogenic or antigenic stimulation. Moreover, it has been shown that inhibition of DPP-IV can suppress the activation of antigen-induced T cell clones and could thus be useful for therapeutic interventions in immune diseases, in particular in autoimmune diseases, such as e.g. MS, and rheumatoid arthritis. Conversely, DPP-IV inhibitors stimulate the production of the immunoregulatory cytokine TGF-β1. See e.g. Reinhold, D. et al. 2000. Cellular Peptidases in Immune Functions and Diseases 2, Langner and Ansorge ed., Kluwer Academic/Plenum Publishers, 155-160; Steinbrecher, A. et al. 2000, Cellular Peptidases in Immune Functions and Diseases 2, Langner and Ansorge ed., Kluwer Academic/Plenum Publishers, 145-153; and Tanaka S. et al., 1997. Int. J. Immunopharmac., vol. 19:15-24.
Currently, several chemical compounds are used in vitro and in animal models to inhibit DPP-IV activity, such as e.g. valine-pyrrolidide (Deacon, C. F. et al., supra), 1-[[2-[(5-cyanopyridin-2-yl)amino]ethylamino]acetyl]-2-cyano-(S)-pyrrolidine (Ahrén, et al., supra), Lys[Z(NO2)]-thiazolidide and Lys[Z(NO2)]-pyrrolidide (Reinhold, et al., supra). However, such chemical compounds have the disadvantage that they often have to be administered by injection, and they may result in side effects as chemical drugs often do.
Proteins, in particular milk proteins, are commonly known as precursors of a range of biologically active peptides. The fact that proteins are precursors of biologically active molecules is particularly attractive for the development of functional foods, such as foods that may aid in any of the above DPP-IV mediated conditions. Food protein hydrolysates are well-used food ingredients and are of natural origin, such that no synthetic ingredients are required for obtaining the functional effect, in case the inhibition of DPP-IV as to prevent or treat DPP-IV mediated conditions such as obesity, type 2 diabetes mellitus and immunological disorders.
Thus, it is desired to provide protein hydrolysates that inhibit DPP-IV activity such that they may aid in prevention and treatment of the above-identified DPP-IV mediated conditions. It was found that protein hydrolysates, such as e.g. milk protein hydrolysates could be used for inhibiting DPP-IV. However, the factors within such hydrolysate that are responsible for such inhibition are as yet unknown. Hydrolysed protein samples are highly complex and can contain up to hundreds of different molecules, which makes it difficult to identify the bioactive compounds in such a sample.